In today's world of increased awareness of the environment along with the high costs and regulations that prohibit and/or limit wastewater disposal to publicly owned treatment services or the environment, there is a demand for water treatment equipment that minimizes wastewater, promotes water reuse in the process, and lowers the quantity of fresh water that has to be imported from wells or public water supplies.
The restraints put on many industries, such as steam-electric power plants, nuclear power plants, and oil production companies, have led to adoption of a Zero Liquid Discharge (ZLD) policy in many instances. A facility can achieve ZLD by collecting and recovering most or all of the water from the wastewater. The resulting highly concentrated wastewater, or dry solids, are then held in ponds on site or the dry solids can be transported to a landfill.
A variety of technologies have been developed to recover water from wastewater or to reduce the volume of the wastewater. These technologies have limitations of complexity and susceptibility to interruption of service or failure of components due to corrosion, fouling, or scaling by the wastewater constituents, especially when feed waters vary from foreseen conditions.
A continuing demand exists for a simple and efficient process which can reliably provide water of a desired quality, in equipment that requires a minimum of maintenance. In particular, it would be desirable to improve efficiency of feed water usage, and lower both operating costs and capital costs for high quality water systems as is required for the various industries.
In most water treatment systems for the aforementioned industries, the plant design and operational parameters generally are tied to final concentrations (usually expressed as total dissolved solids, or “TDS”), which are tolerable in selected equipment with respect to the solubility limits of the sparingly soluble species present. In particular, silica, calcium sulfate, barium sulfate, calcium fluoride, and phosphate salts often limit final concentrations achievable or require operation of the system using the so-called seeded slurry design. To avoid scale formation and resulting decreases in heat throughput, the design and operation of an evaporation based water treatment plant must recognize the possibility of silica and other types of scale formation, and must limit water recovery rates and operational practices accordingly. In fact, typical evaporation plant experience has been that a reduction in distillate flow rates requires chemical cleaning of the evaporator at regular intervals. Such cleaning has been typically required because of scaling, particulate fouling, biofouling, or some combination thereof. Because of the cost, inconvenience, and production losses resulting from such cleaning cycles, it would be advantageous to lengthen the time between required chemical cleaning events as long as possible.
It would be desirable to reduce the scaling, fouling, and corrosion tendencies of the feed water to the point where concentration factors could be increased in the design, and where flux rates could be increased, compared to limits of conventional scale control methods used in water evaporation systems. Raising the allowable concentration factors and flux rates, along with lowering the corrosion potential, is always important to the end user as these design points result in a lowering of capital costs.
Present state of the art embodies several different strategies to alleviate the problems associated with scaling and fouling in higher concentration systems.
These include the use of chelating agents, dispersants, solubility promoters, filters, silica precipitators, operating at low concentration factors, and the use of preferential deposition in a seeded slurry of calcium sulfate (CaSO4) crystals. In the preferential deposition method, the low solubility precipitating crystals tend to deposit on the seeds that are suspended in the circulating solution rather than on the heat transfer surface.
Membrane separation processes have also been used to obtain reusable water from wastewaters but they are typically limited to low recovery operations due to fouling/scaling limits, frequent cleanings, and replacement intervals of three years or less due, in part, to the frequent cleanings which can cause them to lose their rejection capability as well as productivity. A newly patented RO technology, HERON™, utilizes softening and high pH operation to obtain recoveries up to 90 percent but has yet to show an extended membrane life comparable to the 20 years expected of an evaporator. This process is also limited in allowable concentration factor attainable due to osmotic pressure limitations, which currently is around about ten percent total dissolved solids.
The prior art methods have the following shortcomings: (a) they rely on anti-scaling additives to prevent scale formation, or (b) they rely on seeding techniques for preferential deposition to minimize scaling of the heat transfer and other surfaces. Preferential deposition, while it works well in some applications, is not the final answer as it cannot be expected to pick up every individual crystal that is precipitating and some invariably end up on the heat transfer surface, or sump walls, where they themselves then act as a seed site for scale buildup. In addition, certain feed waters do not have enough calcium sulfate (CaSO4) in solution to serve as a self-renewing seed slurry. These feed waters then require the use of additional chemical treatment systems to supply the needed calcium (Ca) or sulfate (SO4), or both, needed for this type of scale control method. Further complications inherent to the preferential deposition method are, (1) the need to carefully control the amount and size of seed that is circulating at any given time as too small a seed will cause fouling to occur in the laminar flow portion of the stream and too much seed will result in plugging of areas like water distribution trays, and (2) there is a limit to the concentration factor obtainable when the presence of double salts, such as glauberite (NaCa(SO4)2) will form scale as the concentration factor is increased.
Thus, for the most part, the prior art methods have one or more of the following shortcomings: (a) they rely on anti-scaling or dispersant additives to prevent scale formation, (b) are subject to scaling, fouling, and a short useful life, (c) they rely on seeding techniques to minimize scale-deposition, or (d) are not able to concentrate beyond 7 or 8 percent TDS. Thus, the advantages of our treatment process, which exploits (a) multi-valent cation removal to non-precipitating residual levels, and (b) efficient dealkalization, to allow extended trouble free evaporator operation at high pH levels, are important and self-evident.
As water is becoming increasingly expensive, or in short supply, or both, it would be desirable to increase the ratio of treated product water to raw water feed in evaporator systems. Therefore, it can be appreciated that it would be desirable to achieve reduced costs of water treatment by enabling water treatment at higher overall concentration factors than is commonly achieved today. Finally, it would be clearly desirable to meet such increasingly difficult water treatment objectives with better system availability and longer run times than is commonly achieved today.
In so far as we are aware, no one heretofore has thought it feasible to operate an evaporator based water treatment system in a scale free environment and at an elevated pH, in continuous, sustainable, long-term operations to produce a high quality water product. The conventional engineering approach has been to design around or battle scale formation, by use of moderate pH, by limiting final concentration factors, by use of chemical additives, or by use of preferential deposition.
In contrast to prior art methods for water treatment, the method described herein uses the essential design philosophy of virtually eliminating any possible occurrence of scaling phenomenon during evaporator operation at the maximum feasible pH, while maintaining the desired concentration factor, and taking the benefit of water recovery that results.